The invention relates to a method for synchronizing a radio communication system divided up into radio cells.
In a cellular radio communication system the necessary multiple use of carrier frequencies in adjacent radio cells gives rise to what is termed “co-channel interference”. In order to reduce this interference, the available carrier frequencies are assigned to individual carrier frequency sub-resources. Each carrier frequency sub-resource is then permanently allocated in each case to a radio cell with the aid of what is termed a “frequency reuse” planning method in such a way that only minimal co-channel interference is caused in the radio cells taking into account minimum physical distances between the radio cells.
Said fixed allocation of carrier frequencies or their transmission resources is disadvantageous in particular when an inhomogeneously distributed number of subscribers occurs in adjacent radio cells. A base station under consideration in one of the radio cells and having to provide coverage to an increased number of subscribers then has an increased demand for transmission resources. If this then gives rise to a lack of transmission resources, then subscribers in the radio cell under consideration who request a new data transmission are rejected.
Accordingly, with an increase in subscriber numbers increased co-channel interference occurs within the radio communication system, which interference can only be influenced to a limited extent by a specified “frequency-reuse factor” on account of the “frequency reuse” planning.
An increase in transmission resources, which is instigated for example in the case of major organized events by subsequent incorporation of further base stations, is not possible in a straightforward simple manner, due to the increase in co-channel interference. Where necessary, the complex and time-consuming “frequency reuse” planning technique must be applied once again.
The use of what are termed “orthogonal frequency division multiplexing” (“OFDM” for short) transmission technologies is gaining increasing significance in particular for future-generation mobile radio networks of cellular design. OFDM mobile radio networks of said type demand high data rates for services such as video transmissions, for instance, which can be transmitted cost-efficiently with the aid of said OFDM transmission technologies. With this approach, a plurality of what are termed “subcarrier frequencies” are used simultaneously in parallel with one another for transmitting a subscriber data stream. A wideband transmission channel is implemented by a plurality of radio transmission channels having a generally identical bandwidth. An OFDM mobile radio network of said kind is in turn to be embodied dependent on a “frequency reuse” planning method that is to be performed with regard to co-channel interference.
The wideband radio transmission channel is “time-dispersive” and is subject to frequency-selective fading, with the result that typically a complex equalization is required on the receive side. In an OFDM transmission the radio transmission channel is subdivided into a plurality of narrower subchannels, with the result that “flat fading” is experienced on each of the subchannels instead of frequency-selective fading, thereby enabling a very simple, typically “single-tap” equalization.
In the simplest case the same modulation scheme, and hence the same transmission bit rate, is assigned each time to each of these radio transmission channels, the assigned transmission bit rates being specified as a function of interference on the respective radio transmission channels. A higher-level modulation method is used for radio transmission channels with low interference than in radio transmission channels that exhibit higher interference. In this manner transmission can be performed with a required quality of service for each radio transmission channel, taking into account an error rate, for example. In the case of a line transmission in the baseband, an OFDM multi-carrier method of said kind is also known under the designation “discrete multitone transmission”, or “DMT” for short.
FIG. 3 shows a cellular OFDM radio communication system according to the related art as a representative instance of all mobile radio systems. Three adjacent radio cells FZ1 to FZ3 each have an assigned base station BTS01 to BTS03. Each individual station of said base stations BTS01 to BTS03 provides coverage to a number of the mobile stations T01 to T012 assigned to the respective radio cell FZ1 to FZ3, with a total of four carrier frequencies f9 to f12 being assigned by a “frequency reuse” planning method to a first base station BTS01 of a first radio cell FZ1, a total of four carrier frequencies f1 to f4 being assigned thereby to a second base station BTS02 of a second radio cell FZ2, and a total of four carrier frequencies f5 to f8 being assigned thereby to a third base station BTS03 of a third radio cell FZ3 exclusively for data transmission.
In a connection direction referred to as the “downlink” DL from the base station to the mobile station, each of the carrier frequencies f1 to f12 has seven timeslots TS1 to TS7 as transmission resources, while in a connection direction referred to as the “uplink” UL from the mobile station to the base station each of the carrier frequencies f1 to f12 has five timeslots TS1 to TS5 as transmission resources. Free, unused timeslots are assigned by way of example to the carrier frequencies f2, f7, and f11 and designated by the letter “F”.
FIG. 4 is an overview showing a synchronization situation of the radio cells FZ1 to FZ3 depicted in FIG. 3 that corresponds to the related art.
The individual base stations BTS01 to BTS03 are neither frequency-nor time-synchronized with one another. A base-station-specific carrier frequency deviation Delta01 to Delta03 is plotted vertically in each case for each one of the base stations BTS01 to BTS03. Said carrier frequency deviation Delta01 to Delta03 is due in each of the individual base stations BTS01 to BTS03 to electrical components of the respective base station, for example to base-station-specific local oscillators. As the mobile stations T01 to T012 are synchronized to the respective assignable base station BTS01 to BTS03, the base stations BTS01 to BTS03 and the correspondingly assigned mobile stations T01 to T012 also have the respective carrier frequency deviations Delta01 to Delta03 with respect to one another.